By Yeshi Wamishe, Extension Rice Pathologist
The rice blast fungus is a worldwide problem of rice that can cause near 100 percent yield loss in susceptible cultivars under favorable weather conditions if left unchecked. The blast fungus is very versatile and effective in its modes of survival. The fungus produces airborne spores that travel long distances, exists in different races that can infect different varieties, survives on rice residues and seeds and infects different parts of a rice plant from seedling to maturity. The major plant parts affected are leaves (Figure 1), collars (Figure 2), nodes (neck) (Figure 3) and panicles (Figure 4). Among these four, early blast can fully kill seedlings, and neck (node) blast (rot) can totally blank rice panicles.
Although early blast can be managed by maintaining deep flood (at least 4 inches), it would be unwise not to plan for fungicide application(s) to suppress neck blast, knowing the variety is susceptible and planted in blast prone conditions. Maintaining deep water until the final drainage time is advised whether the field gets fungicides or not. It may also be a judgement call to apply or not to apply fungicides.
Conditions such as long dew periods (9 hours or more) favor the blast fungal spores to germinate. Dew periods can be prolonged by fog, shade, overcast, and frequent light rains. Late planting, excessive nitrogen fertilizer rates, relatively cooler summer temperatures are in favor of blast disease development. Fields in low lying areas and those that do not hold water or are difficult to water can be blast prone. History of the field, varietal resistance, field management measures, prevailing weather conditions and the presence or absence of early season blast in your field should help you make smart decision regarding fungicide application. Note that the absence of early leaf blast does not guarantee time off for neck blast.
While planning for fungicide applications to effectively suppress neck blast, timing is crucial. Two applications are recommended to obtain up to 95 percent protection provided timing, rate, coverage, and the product are right. Fungicides applied after heads of main tillers are completely out of the boot will serve no use. The first application needs to be made from late boot to 10 percent head out (Figure 5); and the second application, when heads of main tillers are 50-75 percent out of the boot (Figure 6). If the crop is “already headed” and heads of the main tillers are completely out of their boots (Figure 7), it is too late for fungicide application. The most important aspect of this recommendation is the neck should still be in the boot to protect the tender heads from neck blast using fungicides.
Research has shown GEM and Stratego as slightly more effective than Quadris at suppressing neck blast. To protect the crop from neck blast, the higher rates are preferred. Waiting too long to tank mix fungicides for blast with insecticides for stink bugs is not a wise idea. See Table 1 for blast-suppressing fungicides used in Arkansas.